An important class of expert systems is that of diagnostic systems which is characterized by an interactive "consultation" between a user and the system in order to determine the cause of one or more problems associated with a given subject. The system poses a series of questions to the user, these questions typically requiring yes/no responses or responses (such as test value) which the user has determined by observing the subject of the consultation.
Unfortunately, it is sometimes impossible to respond straightforwardly to a given question because the expert system user simply does not know and cannot get the answer to the question. Such a condition may arise, merely by way of example, when the subject is electronics equipment situated at a site remote from the expert system user who is in communication with an individual at the site, and that individual is not sufficiently skilled to provide an answer. Many other examples of circumstances leading to the inability to answer a given question posed by a diagnostic expert system will occur to those skilled in the art.
When the "unknown" response situation has occurred during consultations employing other diagnostic expert systems, some have simply aborted the consultation while others have used rigorous mathematical rules of probability to help make decisions in the face of uncertainty. However, the former is unsatisfactory while the latter may not achieve the highly desirable goal of approximating what a human expert would do when faced with the same dilemma.